(1) Field of the Invention
The present invention relates to a fixing device and an image forming device, and in particular to an electromagnetic induction heating-type fixing device and an image forming device using the same.
(2) Description of Related Art
With the growing demand for energy conservation in the field of image forming devices in recent years, an electromagnetic induction heating-type fixing device having high energy efficiency is attracting attention.
The electromagnetic induction heating-type fixing device includes a fixing rotating body such as a fixing roller and a fixing belt, a magnetizing coil, and a demagnetizing coil. The magnetizing coil is disposed along an axial direction of the fixing rotating body, and generates an alternating field by passage of alternating current to heat the fixing rotating body by induction. The fixing rotating body heated by induction thermally fixes a toner image formed on a passing recording sheet.
The demagnetizing coil is disposed in a non-sheet-passing region of the fixing rotating body through which the recording sheet does not pass, and includes winding wire whose ends are connected together to form a closed loop. With this configuration, the demagnetizing coil generates magnetic flux in such a direction that part of magnetic flux generated by the magnetizing coil and trying to pass the demagnetizing coil is canceled out, thereby suppressing an increase in temperature in the non-sheet-passing region of the fixing rotating body. As a result, thermal degradation of the fixing rotating body and other peripheral members is prevented.
The following patent literatures disclose technology to suppress an increase in temperature in the non-sheet-passing region, in a case where recording sheets of different sizes are used. In such a case, the demagnetizing coil as described above is provided in a plurality so that demagnetization is performed according to a size of the non-sheet-passing region determined when each of the recording sheets passes therethrough.
For example, Japanese Patent Application Publication No. 2009-145421 discloses a fixing device having a structure in which a plurality of demagnetizing coils of different sizes are piled on a magnetizing coil such that a smaller demagnetizing coil is placed on a larger demagnetizing coil (first prior art).
Japanese Patent Application Publication No. 2010-197947 discloses a fixing device having a structure in which a plurality of demagnetizing coils of different sizes are arranged such that a smaller demagnetizing coil is surrounded by a larger demagnetizing coil and that the demagnetizing coils are in a same plane extending along a surface of a magnetizing coil (second prior art). The second prior art differs from the first prior art in that the demagnetizing coils of different sizes are arranged along the surface of the magnetizing coil so as to extend laterally, rather than to be piled on the magnetizing coil.
In the conventional fixing device described above, however, when more demagnetizing coils corresponding to recording sheets of a wider variety of sizes are provided, the following problems arise.
In the fixing device pertaining to the first prior art, since a pile of the demagnetizing coils is placed on the magnetizing coil, the device becomes large in a radial direction of the fixing rotating body. In addition, as the number of piled demagnetizing coils increases, the distance between an upper demagnetizing coil and the magnetizing coil increases and thus demagnetization efficiency is reduced accordingly. Therefore, when a large number of recording sheets corresponding to the upper demagnetizing coil pass and toner images are thermally fixed thereon continuously, over-temperature in the non-sheet-passing region cannot be completely suppressed and thus thermal degradation of the fixing rotating body can occur.
In the fixing device pertaining to the second prior art, since the number of surrounding demagnetizing coils is increased, the width of an outermost demagnetizing coil increases in a circumferential direction of the fixing rotating body and thus the device becomes large accordingly.